As a sample to be inspected, an inspection of a semiconductor wafer will be described as one example. A process in which a pattern formed on a photomask on a semiconductor wafer is transferred by a lithographic processing and an etching processing is repeated to thereby manufacture a semiconductor apparatus. In a manufacturing process of the semiconductor apparatus, the lithographic processing, the etching processing, quality of other various processing items, and foreign substance generation largely exert an influence on a manufacturing yield of the semiconductor apparatus. Accordingly, it is of importance that a pattern on the semiconductor wafer in the manufacturing process is inspected, an abnormality or failure occurrence is detected early or preliminarily and is fed back to a manufacturing process.
As a method for inspecting defects which are present on a pattern on the semiconductor wafer, a comparison inspecting method of acquiring an image by using a apparatus to which an optical microscope or electron microscope is applied, comparing it with an image free of defects, and extracting an image having defects is used.
An inspection apparatus to which an electron microscope is applied has a resolution higher than that of an optical microscope; however, when an electron beam is irradiated onto a semiconductor wafer, a charge phenomenon depending on a pattern material of it occurs. A potential difference on a surface of the semiconductor wafer is reflected on a secondary electron emission efficiency and a potential contrast occurs. By using this, therefore, electrical defects such as conduction and non-conduction of a circuit pattern generated on a surface or in a lower layer, and a short circuit of wiring and transistors can be detected. However, since the entire image may be projected black or white, the charge amount is required to be appropriately controlled. Further, an image contrast between a defect and its surrounding is improved, and a surface potential is stabilized and brightness of the image is uniformed. For that purpose, with regard to precharge for irradiating electrons onto the semiconductor wafer before acquiring an inspection image, whether a precharge result becomes equal to a desired charge amount is required to be known.
An operator acquires an image and visually determines whether precharge conditions are appropriate. Even if a desired image is obtained by repeating the above, the operator has a defect of requiring much time until final setting of the precharge conditions. In addition, when an electron beam is irradiated onto the same portion many times, a charge state changes. Accordingly, since the operator waits for relaxation of the charge after the irradiation and then repeats the irradiation, it takes more time to set the precharge conditions. Further, in the case where one insulting film on a bottom surface or another insulating film on a sidewall of the semiconductor wafer is thick, when the precharge is performed, an image is saturated to be white. In the case of comparing and inspecting the image of this state and an image having the same state as that of the above image, a gradation difference fails to be extracted even if a defect is present, and therefore, it is determined to be free from defect. As described above, the inspection apparatus plays a role in monitoring the presence or absence of defects in a semiconductor manufacturing line; however, when the precharge is not appropriate, reliability of the inspection is lowered.
Accordingly, since it is of great importance that prior to an inspection, a charged state of a semiconductor wafer is known, various methods are proposed. For example, one method is disclosed in which a relationship between an image as a standard sample and the charge amount is previously obtained, and then the above image and the acquired image are compared to thereby estimate the charge amount (see, for example, Patent Literature 1). Further, another method is disclosed in which a relationship between a resistance value of a semiconductor wafer, a potential contrast image, and a charge voltage is obtained, and then the resistance value is measured to thereby determine the charge voltage from the potential contrast image (see, for example, Patent Literature 2). In addition, another method is disclosed in which a potential is measured by using electrodes provided on a surface of a semiconductor wafer (see, for example, Patent Literature 3). However, with regard to the method for previously obtaining a relationship between the image and the charged state, and then estimating the charge amount from the image acquired prior to an inspection, it is doubtful whether it indicates its real charged state. The reason is that the charged state delicately changes due to a difference of a shape or material of a sample and irradiation conditions of electron beam. Further, with regard to the method for measuring a potential by using the electrodes provided on a surface, since a potential of a position distant from the sample is measured, the charged state of the sample itself is not measured.